How I Met Your Mother met its end Monday night. And many fans are giving the series finale of the wildly popular CBS sitcom a big thumbs down. But they still watched in droves: The one-hour episode averaged 12.9 million viewers, according to early Nielsen estimates, marking the most-watched episode of its nine-year run.

Ted Mosby (Josh Radnor) finally met "the Mother" —Tracy McConnell, played by Cristin Milioti — and proceeded to have babies, get married, and eventually sit by her side as she got sick and died.

After waiting nine years for a happy ending, fans didn't get it.

Vulture called it "a slap in the face." Variety said it was "fine, if not, alas, legendary. But hey, unlike some of the shows that have come and gone while Ted searched for his dream girl, it certainly beat a slap to the face." Time's reviewer wrote, "Me, I'll be mad at this finale for a while, but in the end, it was a bad hour of TV, at the end of many, many good half hours of TV."

Twitter erupted "with rage," notes Us Weekly, soon after the show ended - and hasn't stopped. Some felt disappointed, some felt betrayed.